Prevent double acking of ignored touches

ui/aura/window_event_dispatcher.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/aura/window_event_dispatcher.h"
 
 #include "base/bind.h"
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
@@ skipping 875 matching lines @@
     default:
       NOTREACHED();
       break;
   }
 
   if (dispatching_held_event_ || !held_move_event_ ||
       !held_move_event_->IsTouchEvent()) {
     ui::TouchEvent orig_event(*event, target, window());
 
     // If the touch event is invalid in some way, the gesture recognizer will
-    // reject it. In this case, stop the touch from reaching the next event
-    // phase.
+    // reject it. This must call |StopPropagation()|, in order to prevent the
+    // touch from being acked in |PostDispatchEvent|.
     if (!ui::GestureRecognizer::Get()->ProcessTouchEventPreDispatch(orig_event,
                                                                     target)) {
-      event->SetHandled();
+      event->StopPropagation();
     }
   }
 
   PreDispatchLocatedEvent(target, event);
 }
 
 }  // namespace aura